1. Field of the Invention
Embodiments of the invention generally relate to methods for depositing silicon-based materials. More specifically, the present invention relates to chemical vapor deposition techniques for fabricating silicon nitride stacks.
2. Description of the Related Art
Manufacturing techniques for fabricating devices on integrated circuits are continually evolving to facilitate the continued shrinkage of the devices formed on the integrated circuit and their more dense arrangement on the substrate. For example, in order to prevent current leakage, each transistor formed on the substrate was traditionally isolated from adjacent transistors through a localized oxidation of silicon (LOCOS) field oxide process. This same function may now be performed using a shallow trench isolation (STI) process to isolate each transistor from its adjacent transistor. The shallow trench isolation takes up less space on the substrate while performing the same function as LOCOS, which in turn increases transistor density on the chip.
However, the STI process creates its own manufacturing challenges. For example, as part of the fabrication process to form an STI structure, a silicon nitride film is typically formed on the substrate by a furnace process utilizing dichlorosilane and ammonia precursors. The fabrication of the thick films (typically about 1500 Angstroms) typically utilized in the STI fabrication process utilizing the standard furnace process results in the deposition of large amounts of material on the furnace walls. The high rate of furnace wall deposition requires greater frequency of preventative maintenance to avoid excess particle generation, thereby lowering productivity and increasing the risk of defects due to particle contamination.
Moreover, furnace films are deposited on both sides of the substrate—the back side silicon nitride film being stripped after patterning the front side of the substrate to make the STI structures. This process results in a high stress level in the silicon nitride film remaining on the substrate, which may cause dislocations to form in or near the STI structures. The dislocations in the substrate or in the STI structures may lead to increased electrical leakage of the transistors disposed proximate the STI structures due to electron hopping along those dislocations. An alternative method using single-wafer chamber and silane-ammonia chemistry for forming silicon nitride film, while meets stress and particle requirements, does not meet requirements of film uniformity.
Thus, there is a need in the art for an improved method for fabricating a silicon nitride stack suitable for use in fabricating a shallow trench isolation structure.